The primary goal of the project is to determine the behavioral, ecological and reproductive factors which characterize a population of house mice (Mus musculus) at each of several successive stages of crowding. The rate of population growth has been regulated: Once the adult population reached 100, the population was allowed to double approximately every 200 days. After 1200 days the population reached about 1800 and then began to decline because the death rate exceeded the rate of recruitment. At present, although many pups are born each day, none are reared to the age of weaning. If this lack of effective reproduction continues for another 200-400 days (until the youngest generation becomes reproductively senescent) the population will decline to extinction.